In recent years, a number of memory chips, such as flash memory chips, have been stacked on top of each other in order to meet the demand for an increase in the capacity of semiconductor memories. With regards to this, in the case where signals are transmitted and received between chips via bonding wires, a problem arises such that the number of bonding wires is tremendous, which restricts the number of chips stacked on top of each other.
Therefore, the present inventor has proposed an electronic circuit where communication is carried out between IC chips that are stacked on top of each other through inductive coupling via coils formed of wires on the chips (see Patent Documents 1 to 7 and Non-Patent Documents 1 to 9).
The use of this technology makes it possible for data to be sent and received between a number of IC chips through wireless communication. As a result, wires for data communication become unnecessary, and therefore, the number of bonding wires can be reduced and the number of chips that are stacked on top of each other can be increased.
Non-Patent Documents 8 and 9 report that 64 NAND flash memory chips and one controller chip are stacked on top of each other for the communication between chips through inductive coupling where the controller can access any of the NAND flash memory chips at random, for example. As a result, it is possible to implement a solid-state drive (SSD) with a large capacity within one package.
In this case, coils used for the communication through inductive coupling are formed of wires on the IC chips. Though the distance for communication may slightly vary depending on the performance of the communication circuit, wireless communication over the distance that is approximately ⅓ to 1/1 of the diameter of the coils becomes possible.
In Non-Patents Documents 8 and 9, for example, communication is carried out over a distance of 120 μm using coils having sides of 200 μm. In this example, the thickness of one IC chip is 60 μm, including the thickness of the adhesive, and therefore, communication between every other IC chip is possible by bypassing one IC chip. Accordingly, data communication is possible between any chips desired from among the 64 chips by repeating data transfer one after another between chips that are close to each other in the upward or downward direction.